In both cases, though, they weren't meant to be "guest" characters, but permanent add-ons to the cast, and the commentaries seem to indicate that it was network interference.
Having said that, they still had some great moments - I love Jimmy's explanation to Dave why he would be a terrible rolemodel for nephew Walt.

We get another great "Hey, I'm a character obviously forced in by the network!" at the end of Season Four.
I love how they give them the ol' Poochie sendoff, if they even bother to mention them at tall.

Thank you, I think I will.

It also puts Matthew is a wholly less-sympathetic light - he's not, in fact, a complete idiot but rather pursues an occupation that no only is he completely unsuited for, but doesn't really try very hard at either.

Wait until later in the season. Or, I guess, go ahead and watch them.

There's also the later episode in Season Four where Bill attempts numerous (and to we viewers, hilarious) impressions that no one gets, and then is upstaged by a guy who really sucks at it.

The only thing dead about Jesse is his showbiz career. Ba-zing!

Man, next you're going to spoil the ending of Jack Black's turn as Gulliver.

Careful, or maybe he'll pull a "Lost" and GRRM will say "What happens? They all die. Eventually. Don't worry about all those plots, the story was really about friendship and love."

Why would he think that someone is coming to take him away, then?

Ah, Bloom County!

There's a lot more subtle humor in the earlier episodes.

Comedy Central better have Homeland Security out in force to protect the return of Futurama, then.

I'm sorry "Ktotwf", you know Official Hipsters can't grasp that the X-Files was really about LOVE and FRIENDSHIP. Chris Carter obviously answered all the important questions.

Yeah, I actually like the Lone Gunman show. Much like Millenium, the X-Files managed to give another cancelled show a completely underwhelming final sendoff.

There's an old, old one from 1998 that I got for the PC (also for Mac and the PS1 according to Wikipedia) - it's a FMV game where you play a Seattle-based FBI agent, and the various X-Files regulars are extras in your story. (Mulder and Scully have gone missing in the area, and Skinner requests help from the local…

Actually
Speaking of tie-ins, the X-Files video games were pretty cool. In many ways, they were better than a lot of the later seasons.

"Fearful Symmetry" is an exemplar of the problem with the two sides of the X-Files - 'monster-of-the-week' versus 'the conspiracy', which were pretty much two unrelated shows. (My gf prefers the former, I the latter). At the time, there would be discussions how eps like this, or the awful Mars one from the first…

The first twelve issues of the X-Files comic told a surprisingly good story.

Jefferson Twilight certainly thinks he looks evil.
But as the Alchemist pointed out - he saves people money!